Why do the keyframes show up in the dopesheet but not in the timeline and the graph-editor?
The keyframes you're looking for are Grease Pencil Keyframes, and they have their own interface, which is the dopesheet grease pencil panel.
In the timeline, ordinary dopesheet and graph editor will appear only for keyframes related to the Grease pencil object as a whole, while those of yours are related to every single vertex of GP strokes, and they are not editable in the graph editor.
When you create the interpolation between two extremes, you can choose between linear, the preset of easing in and out, or even define a custom curve of interpolation.